Abstract

BACKGROUND:

Opiates such as morphine are the most powerful analgesics, but their protracted use is restrained by the development of tolerance to analgesic effects. Recent works suggest that tolerance to morphine might be due to its inability to promote mu opioid receptor endocytosis, and the co-injection of morphine with a mu opioid receptor internalizing agonist like [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]enkephalin reduces tolerance to morphine. So far, no studies have been conducted to evaluate the ability of methadone to reduce morphinetolerance in morphine-pretreated animals, a treatment sequence that could be encountered in opiate rotation protocol. We investigated the ability of methadone (a mu opioid receptor internalizing agonist used in therapy) to reverse morphinetolerance and the associated cellular mechanisms in the periaqueductal gray matter, a region involved in pain control.

METHODS:

We measured analgesic response following a challenge dose of morphine in the hot plate test and investigated regulation of mu opioid receptor (coupling and endocytosis) and some cellular mechanisms involved in tolerance such as adenylate cyclase superactivation and changes in N-methyl-d-aspartate receptor subunits expression and phosphorylation state.

RESULTS:

A chronic treatment with morphine promoted tolerance to its analgesic effects and was associated with a lack of mu opioid receptor endocytosis, adenylate cyclase overshoot, NR2A and NR2B downregulation, and phosphorylation of NR1. We reported that a methadone treatment in morphine-treated mice reversed morphinetolerance to analgesia by promoting mu opioid receptor endocytosis and blocking cellular mechanisms of tolerance.

CONCLUSIONS:

Our data might lead to rational strategies to tackle opiate tolerance in the frame of opiate rotation.

Effects of methadone on mu opioid receptor (MOPr) coupling in morphine-treated mice. Animals were treated with i.p. injection, twice daily, for 4 days with morphine (10mg/kg) followed by 1 day of treatment with saline (Morph/Sal), morphine (Morph), methadone (2.5mg/kg, Morph/Meth), or for 5 days with saline (Sal). On the sixth day, periaqueductal gray matter (PAG) was extracted and [35S]-GTPγS binding was measured in the presence of increasing concentration of D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin (DAMGO). Plotted values represent percentage of basal [35S]-GTPγS binding (without agonist), as mean±SEM, n= 3 to 4 determinants/group.